my husband did a 2 week water/coffee fast a few years ago. (he drank one or two cups of coffee each day because he didn't want to give that up). He was in his late forties at the time and has fm. At first I was worried, but he seemed to do just fine. He didn't seem weak, but then he was 25 years younger than your mom.
He said that after the first couple of days he felt great. Very light, clear headed and much less pain. Every now and then he talks about doing it again, but has yet to do it.
Regarding grapes, I would recommend only organic since they are in the "dirty dozen" (foods that you should try to buy organic as much as possible, because they contain the most pesticide residues).
If my husband were to fast again, I would recommend that he keep a food diary once he started eating again and introduce foods one or two at a time. Since he felt so much better on the fast, I suspect that something in his diet may contribute to his symptoms. However, he just doesn't want to spend time or effort to investigate this. Hard for me to believe...